1. Field of the Invention
The present invention relates to an optical waveguide for use in optical interconnections and the like, and a method of manufacturing the optical waveguide.
2. Description of the Related Art
Recent years have seen a remarkable progress of the optical communications technology. It has been proved that the optical communication is advantageous over the electric communication. As the signal-processing speed in LSI and the like has increased, techniques for replacing electric signals with optical signals are being developed. It is expected that media for transmitting optical signals will be polymer optical waveguides that have been developed in recently years.
The polymer optical waveguide can be formed to have a large area. Attempts have been made to apply the polymer optical waveguide to optical interconnections of the order of 1 cm to 1 m. The polymer optical waveguide may have, at one end, an optical-path changing mirror. This makes it possible to mount optical components on a surface just above the optical-path changing mirror.
(Method of Manufacturing the Waveguide)
The polymer optical waveguide is manufactured, generally by a method that uses dry etching as shown in FIG. 44 or by a method that utilizes pattern exposure and development as shown in FIG. 45.
More specifically, in the method using dry etching, a first clad 2 is formed on a substrate 50 and a core 1 is formed on the first clad 2, as is illustrated at (a) in FIG. 44. As depicted at (b) in FIG. 44, a silicon-containing resist 51 is formed on a part of the core 1. As shown at (c) in FIG. 44, reactive ions 52 are applied to the silicon-containing resist 51 and the core 1, thereby etching that part of the core 1 which is not covered with the silicon-containing resist 51. As shown at (d) in FIG. 44, the silicon-containing resist 51 is removed, forming a core 1 projecting upwards. As depicted at (e) in FIG. 44, a second clad 3 is formed on the projecting core 1 and the first clad 2.
In the method utilizing pattern exposure and development, a first clad 2 is formed on a substrate 50 as shown at (a) in FIG. 45, and a core material 1′ is formed on the first clad 2 as illustrated at (b) in FIG. 45. As shown at (c) in FIG. 45, ultraviolet rays are applied to the core material 1′ through a photo mask 35, thus curing a part of the core material 1′. As depicted at (d) in FIG. 45, that part of the core material 1′ which has not been cured is removed by means of development, forming a core 1 that projects upwards. As shown at (e) in FIG. 45, a second clad 3 is formed on the projecting core 1 and the first clad 2.
The optical-path changing mirror is formed, as in most cases, by a mechanical process that uses a dicing saw as illustrated in FIG. 46. In the mechanical process using a dicing saw, a substrate 50 is prepared as shown at (a) in FIG. 46. The substrate 50 has clads 2 and 3 in which a core 1 is embedded as is illustrated at (e) in FIG. 44 or (e) in FIG. 45. As shown at (b) in FIG. 46, both ends of the core 1 are cut slantwise with a dicing blade 54. At the same time, the clads 2 and 3 are cut slantwise with the dicing blade 54. As a result, both ends of the core 1 make total-reflecting mirrors 55 as depicted at (c) in FIG. 46. At this time, an optical path is formed, through which signal light 8 applied to one end of the core 1 passes until it emerges from the other end of the core 1.
The waveguide shown in FIG. 44 or FIG. 45 and the optical-path changing mirror shown in FIG. 46 are manufactured in separate processes. Inevitably, the manufacture of the system is complex and requires a high cost.
To manufacture the waveguide and the mirror at the same time, a method using a mold has been devised (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-154049, pages 8 and 9, FIGS. 2 and 3). In the method using a mold, the entire surface of a substrate that has a recess is coated with a core. The core is then removed from the substrate, but not from the recess. A first clad is formed on the entire surface of the substrate, covering the core remaining in the recess. The core and the first clad are transferred onto a separate substrate. Thereafter, a second clad is formed on the first clad.
In this method, the core applied to the entire surface of the substrate is removed, but not from the recess. The use efficiency of core material is therefore low. The cost of the method is high.
A method in which the core material is used at high efficiency is available (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-90544, page 7, FIGS. 1 to 5). This method uses a recessed mold that is transparent to light and has a light-shielding film on its surface, but not over the recess. Hence, light is applied through the recessed mold, curing only the core pattern. However, the recessed mold, which is made of resin, will likely be deformed by temperature deviation. The core pattern is inevitably deformed.
A similar technique is disclosed in W. J. Oh, M. S. Kim, H. H. Byum, J. W. Kim, K. S. Han, J. H. Oh, M. S. Kwon and S. Y. Shin, “Fabrication of Multimode Polymer Optical Waveguides by Using UV Curable Resins and Transfer Molding Process,” Seventh Optoelectronics and Communications Conference (OECC 2002), Technical Digest, pp. 534-535, July 2002. This technique uses light applied through a recessed mold, too; the thesis reads, “The PDMS mold is transparent to UV light (page 534, right column, lines 11-12).” Since light is applied through the recessed mold, the mold made of resin is inevitably deformed.
(Mounting of an Optical Component)
The optical waveguide has a core on which an optical-path changing mirror is provided. An optical component, which is a light-emitting element or a light-receiving element, is mounted on the surface of the optical waveguide lies on the optical axis of the mirror.
In most cases, the optical-path changing mirror is a plane mirror. The plane mirror is disadvantageous in that the connection efficiency is low when it guides light to the core from a light-emitting element such as a vertical-cavity surface-emitting laser (VCSEL) or to a light-receiving element such as a photodiode (PD). The plane mirror is disadvantageous also in that the displacement tolerance is small.
To connect the light-emitting element to the core, a convex lens is used, as in most cases, to convert the diverging light coming from the light-emitting element to focused light, which is applied to the optical-path changing mirror. To connect the core to the light-receiving element, a convex lens converts the light coming from the optical-path changing mirror to focused light, which is applied to the PD, in order to increase the connection efficiency and the displacement tolerance for the light-receiving element (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-185752).
In these methods, however, it is necessary to provide an optical path between the core and the convex lens, which is longer than the diameter of the core. This inevitably renders the entire system large and complex. Further, the medium outside the lens must be one having a small refractive index, and air is usually used. Thus, no highly reliable structure, such as a transparent resin capsule, can be used.
Both the light-emitting element 40 and the light-receiving element 41 may be provided near optical-path changing mirrors 4 and 6, as shown in FIG. 47, establishing the relation of the diameter of beam emitting area<the diameter of the core<the diameter of beam receiving area. Thus, the light beam reaches the light-receiving element 41 before it greatly diverges. This renders it unnecessary to use the convex lens. This method is not so desirable, however. The light beam receiving area has a large diameter, and the light-receiving element 41 can respond but slowly.
(Mounting of the Waveguides)
As FIG. 48 shows, straight waveguides, curved waveguides and inclined mirrors, each at the end of any waveguide, have been hitherto used (see, for example, p. 662, FIG. 8, Journal of the Society of Electronic Data Communication, Vol. 84, No. 9, pp. 656-662, September 2001). Straight waveguides are fundamental. A curved waveguide is used to change the position or orientation of a straight waveguide. Incline mirrors are used to connect waveguides to surface-emitting elements or light-emitting elements (hereinafter, referred to as “external elements”).
Many cores are required in complex circuit. In a complex circuit, it is difficult to amount straight waveguides and curved waveguides in high density. This is because each curved waveguide cannot have a small radius of curvature; the smaller the radius of curvature, the greater the loss of light. Since the curved waveguides need to have a large radius of curvature, a large area is required to change the direction of the optical path. It is therefore difficult to increase the density at which the waveguides may be mounted.
Further, in complicated circuits, the setting must be repeated many times to process mirrors by laser cutting.
In summary, any structure comprising straight waveguides, curved waveguides and inclined mirrors, each provided at the end of each waveguide, is not considered to be fit for providing cores that connect many points at various positions.
(Bonding to Another Substrate)
How an optical waveguide 7 is made in the form of a film and bonded to another substrate will be described below.
How a film, or optical waveguide 7, is formed as is illustrated in at (a) to (f) in FIG. 49. As shown at (a) in FIG. 49, a first clad 2 is formed on a substrate 20. As depicted at (b) in FIG. 49, alignment marks 79 are formed on selected parts of the first clad 2. Then, as shown at (c) in FIG. 49, a core 1 is formed on the first clad 2, not overlapping the alignment marks 70. At (c) in FIG. 47, the core 1 is depicted as if overlapping the alignment marks 70. However, the core 1 is displaced from the marks 70 in the direction perpendicular to the plane of the drawing. As shown at (d) in FIG. 49, a second clad 3 is formed on the core 1 and the first clad 2. The waveguide 7 is thus provided on the substrate 20. Thereafter, as shown at (e) in FIG. 49, inclined, total-reflection mirrors 55 are formed at the ends of the core 1. The substrate 20 is peeled off the optical waveguide 7. The optical waveguide 7, shaped like a film as depicted at (f) in FIG. 49, is therefore manufactured.
Next, as shown at (g) in FIG. 49, the optical waveguide 7 is bonded with adhesive 62 to another substrate (e.g., a wiring board) 60, with one alignment mark 70 aligned with alignment marks 61 that are provided on the substrate 60. This completes the bonding of the optical waveguide 7 to the other substrate 60.
This bonded structure can hardly be controlled, however, in the thickness of the adhesive layer 62. The distance between the optical waveguide 7 and the other substrate 60 may change in accordance with the thickness of the adhesive layer 62. Further, the precision of positioning the waveguide 7 with respect to the substrate 60 is low because the alignment mark 70 is spaced apart from the alignment marks 61 by a long distance.